1. Field of the Invention
The invention relates to amplifiers in general and more particularly to devices and techniques that improve the operating characteristics of such amplifiers.
2. Prior Art
Transistor amplifiers are widely used in different types of electronic equipment. A fundamental tradeoff in the design of an amplifier is that of power consumption versus performance. A typical transistor amplifier has at least an input stage which accepts an input signal, one or more intermediate stages and an output stage which delivers the input signal to a load circuit. Power for the amplifier is supplied from a power source connected to the amplifier.
The output stage of the amplifier frequently operates in what is known as the large signal region of operation. In this region of operation the signal or load current is the same order of magnitude as the quiescent or bias current. When an amplifier is operating in this large signal mode, many of its characteristics vary as a function of the signal level. For example, some of the critical performance characteristics which vary with operating currant include output impedance, bandwidth and linearity.
Even though the above problem occurs in several types of amplifier topologies, its effect is noticeable in a common emitter follower configuration which is used to illustrate the problem. FIG. 1 shows an electrical schematic of a prior art emitter follower amplifier operating in a class A mode. When the output voltage (V.sub.o) rises towards a maximum, the emitter current (I.sub.e) of the transistor is Ilmax.+Iq. When the output voltage decreases towards 0 volts, Il decreases and the emitter current of Q'1 approaches Iq. As the output voltage goes negative, the load current (Il) goes negative and the emitter current of Q'1 decreases towards 0.
The result of this large signal operation is that Q'1 collector current could vary from Iq +Ilmax. to no current at all. Since many transistor parameters are a function of collector current, the performance of the amplifier can vary significantly with signal levels and is thus non-linear.
The prior art has provided several techniques to compensate for the non-linearity in amplifiers. One obvious technique is to increase Iq in order to have it dominate over Il variations. This technique would improve linearity at the expense of power consumption.
Another prior art technique used for improving the non-linear characteristics of amplifiers is negative feedback. Even though this technique works well for its intended purpose, it has drawbacks which adversely affect the operation of the amplifier. The ability of negative feedback to improve performance is limited by the amount of excess gain available as well as stability limitations. Feedback is also limited in its ability to improve large signal performance by the dynamic range of error correcting circuitry within the feedback loop.
Cascode topologies are another prior art technique used to improve the non-linear characteristics of amplifiers. Cascode circuit topologies are commonly used to improve amplifier performance by adding transistors which maintain a primary amplifying transistor at a constant operating voltage. This technique can be used to widen bandwidth and improve linearity in voltage gain amplifiers. However, Cascode techniques are limited in their application to current only gain amplifiers. In addition, Cascode circuitry also suffers from a reduction in signal swing due to voltage drops required for the additional Cascode devices.